meridional overturning circulation: some basics and its multi-decadal variability · 2013-06-21 ·...
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MERIDIONAL OVERTURNING CIRCULATION: SOME BASICS AND ITS MULTI-DECADAL
VARIABILITY Gokhan Danabasoglu
National Center for Atmospheric Research
OUTLINE:
- Describe thermohaline and meridional overturning circulations,
- Multi-decadal variability in the North Atlantic as depicted by the Atlantic Meridional Overturning Circulation (AMOC),
- Examples of climate impacts and potential predictability,
- Results from the NCAR Community Climate System Model (CCSM3) simulations,
- Summary
WHAT IS THERMOHALINE CIRCULATION (THC)?
It is that part of the ocean circulation which is driven by density differences (as opposed to wind and tides).
Because the ocean density is a function of temperature (thermo) and salinity (haline), this circulation is referred to as the themohaline circulation and indicates a driving mechanism.
These density differences are primarily caused by surface fluxes of heat and freshwater and subsequent interior mixing.
The oceanic density distribution is itself affected by the currents and associated mixing. Thermohaline and wind driven currents interact with each other, and therefore cannot be truly separated.
THC IS NOT AN OBSERVATIONALLY MEASURABLE QUANTITY!
THERMOHALINE CIRCULATION PATHWAYS
“CONVEYOR BELT”
While temperature acts as the driver, salinity provides the break!
WHAT IS MERIDIONAL OVERTURNING CIRCULATION (MOC)?
It is a related field, referring to a streamfunction on the depth-latitude plane. It can be obtained from
where x: longitudinal (zonal) direction (+ve eastwards) y: latitudinal (meridional) direction (+ve northwards) z: height (+ve upwards) t: time V: meridional velocity component
This field is often used in the modeling community, because it is easy to diagnose.
MOC INCLUDES WIND-DRIVEN CIRCULATION!
MERIDIONAL OVERTURNING CIRCULATION
GLOBAL
ATLANTIC
Units: Sverdrup
(Sv =106 m3 s-1)
NORTH ATLANTIC DEEP WATER (NADW)
ANTARCTIC BOTTOM WATER (AABW)
WIND DRIVEN
MERIDIONAL OVERTURNING CIRCULATION
80oS 40oS Eq. in Sv
Depth – Latitude Plane Density – Latitude Plane
OCEANIC NORTHWARD HEAT TRANSPORT
Trenberth and Caron (2001)
SEA SURFACE TEMPERATURE SEA ICE CONCENTRATION
WHAT DRIVES THC / MOC?
MECHANISM I: Cooling at high latitudes. For steady state, downward penetration of heat by mixing is necessary.
NORTH SOUTH
surface cooling
mixing
low density
high density
Turbulent mixing supplies energy.
-z
WHAT DRIVES THC / MOC? MECHANISM II: Westerly winds over the Southern Ocean. No meridional flow can be supported at intermediate depths at the latitude band of the Drake Passage due to lack of topographic barriers that can support east-west pressure gradients.
NORTH SOUTH
low density
high density
Winds directly supply energy.
-z
westerlies
EQUATOR
Many coupled general circulation models (CGCMs) exhibit multi-decadal or longer time scale (20 - 100+ years) variability in their AMOCs.
Bryan et al. (2006, J. Climate)
Time series of the AMOC maximum from CCSM3 present-day control simulations
T85x1
T42x1
T31x3
Gent et al. (2006, J. Climate)
HEAT CONTENT CHANGES between mid-1990s and mid-1950s
(CCSM3 20th Century simulations – 1870 control integration)
L: Levitus et al. (2005) observational estimates
Tom Delworth
CHANGE IN SOME FIELDS BETWEEN HIGH AND LOW AMOC PERIODS IN THE GFDL CM2.1 CONTROL SIMULATION
Rainfall (cm day-1) Vertical shear of zonal wind (m s-1)
Vertical shear computed for 300 hPa – 850 hPa.
AMOC IN THE 20th CENTURY ENSEMBLE INTEGRATIONS
PI CONTROL
Frank Bryan
Time series of AMOC maximum from 5 members of a 30-member ensemble of CCSM3 (T42x1) A1B scenario simulations
28 22
14 Sv / time interval
Bryden et al. (2005)
ECMWF
TIME SERIES OF NORTH ATLANTIC MOC AT 25oN
(4-35 Sv) RAPID Array
ATLANTIC MULTI-DECADAL OSCILLATION (AMO)
Sutton & Hodson (2005)
Trenberth & Shea (2006)
AMO INDEX (SST, oC)
SST vs AMO INDEX REGRESSION (oC/SD)
AMO INDEX
Since Delworth et al. (1993) study, there is a broad consensus that the density anomalies in the “sinking region” of the AMOC drives this variability.
However, many fundamental questions still remain largely unanswered:
- mechanism [nature of this mode, role of atmospheric variability],
- robustness of mechanism,
- time-scale,
- implications for initialization (and predictability),
- implications for our assessments of 20th century, future scenario, etc. climates, - ……
QUESTIONS
AMOC IN CCSM3 T85x1 RESOLUTION, PRESENT-DAY CONTROL SIMULATION
Danabasoglu (2008, J. Climate)
SEA SURFACE TEMPERATURE (SST)
oC
99%
NORTH-SOUTH GYRE BOUNDARY FLUCTUATION and WIND STRESS CURL SIMULTANEOUS REGRESSION
x10-8 N m-3 / o lat
MARCH-MEAN BOUNDARY LAYER DEPTH (BLD)
BLD and AMOC PC1 time series
DENSITY REGRESSIONS WITH AMOC PC1 TIME SERIES
99%
AMOC leading
AMOC lagging AMOC leading
m
-10 -5 0 5 10
WIND STRESS CURL
x10-8 N m-3
99% 95%
99% 95%
SUMMARY
• Although they refer to different concepts, THC and MOC are often used as synonyms.
• There are no long-term observational estimates of the MOC transport.
• Many CGCMs exhibit multi-decadal or longer time scale variability in their AMOCs.
• This variability is usually associated with variations in the ocean heat transport, ocean heat content, North Atlantic SSTs (e.g. AMO), climate changes over North America, Western Europe, and Africa.
• There are indications of potential predictability.
IN CCSM3 T85x1 RESOLUTION, PRESENT-DAY SIMULATION:
• This multi-decadal variability shows rather large amplitudes in both AMOC and SST. Comparisons of the latter with observations indicate that neither the pattern nor the magnitude of the SST anomalies is realistic. However, the role of the mean-state biases remains unclear.
• These SST anomalies are created by the fluctuations of the subtropical -subpolar gyre boundary driven by small scale WSC anomalies.
• The present results do not support an ocean mode that relies on a phase lagged relationship between temperature and salinity in their contributions to the total density in the model’s associated deep water formation region.
• Atmospheric variability associated with the model’s NAO appears to play a prominent role in maintaining this variability.
• It is likely that the processes setting the 21-year time scale have oceanic origins.
ATLANTIC NORTHWARD HEAT TRANSPORT (NHT)
PW / AMOC PC1 variance
LABRADOR SEA ADVECTIVE HEAT FLUX REGRESSIONS WITH AMOC PC1 TIME SERIES
S
E+N
LAB
m
OBS: Levitus et al. (1998) & Steele et al. (2001)
BAROTROPIC STREAMFUNCTION
Sv
oC
MEAN SST BIAS
0 -15 -10 -5 +5
max AMOC
“max” SST
max density
BLD
min AMOC
positive NAO
negative NAO
strong subpolar gyre
- reduced sea-ice cover, - increased surface heat loss, - increased upwelling of salt
- increased sea-ice cover, - reduced surface heat loss, - reduced upwelling of salt, - diffusive fluxes
SIMPLIFIED DIAGRAM OF PHASE RELATIONSHIPS